This invention relates to asynchronous data receivers and especially to such receivers for use in optical data links.
Fiber-optic data links have been proposed as a substitute for coaxial and other wired links to interconnect a variety of electronic equipments such as computers and the like. In the No. 4 Electronic Switching System manufactured by Western Electric Company, such data links may one day interconnect time slot interchangers with voice interface frames, time multiplexed switches with clock circuits, and peripheral unit buses with the No. 1A processor and the time multiplexed switches.
These fiber-optic data links are being given serious consideration as replacements for coaxial cable links for a number of reasons: optical signals offer wider bandwidth and are immune to electromagnetic interference; and fiber-optic interconnections electrically isolate the interconnected equipment on different frames, and reduce the cable congestion on frames.
Ideally, an optical data link, in some applications, should have an input-output characteristic which is completely independent of the input data format; that is, the data link should not be sensitive to how often, or how infrequently, data pulses occur. In addition, the link should not depend on a fixed data pulse width or on a clocked stream of data pulses.
Usually, an optical transmitter encodes a data signal into binary, or two-level, optical signal where the light from a junction laser or LED, for example, is modulated between zero (or near zero) intensity and some predetermined peak light amplitude in accordance with information to be transmitted. This modulated light signal is then processed in a linear fashion (detected, amplified, filtered) through an optical receiver up to the point of a threshold detection circuit. The optical data signal can be decoded either by detecting the light amplitude and using it to set a threshold level or by detecting the zero crossings of the signal in the case that the duty cycle averages 50%. But, neither of these techniques can be made to work well with very infrequent data (e.g., duty cycles of less than a few percent). A typical prior art solution to this problem in a clocked system is to scramble the data at the transmitter in such a way that a 50% duty cycle results and then to unscramble it in the receiver. Another approach uses various coding schemes such as Manchester coding. In this type of coding every data pulse interval of duration T is converted to a data pulse of duration T/2 in the first half of the time slot and no data pulse in the last half of the time slot, or conversely. The (10) coding would correspond, for example, to the presence of a data pulse of duration T, whereas the converse (01) coding would correspond to absence of a data pulse in the interval T. In unclocked (asynchronous) systems, however, these methods do not work.
Instead, asynchronous systems often utilize a three-level transitional coding scheme; that is, at the transmitter each transition of an input binary electrical data pulse of duration T (FIG. 1) is coded into an electrical pulse of duration .tau.&lt;T, which in turn is used to modulate the light amplitude of a laser or LED light source, thus generating a transitionally encoded, three-level optical signal (FIG. 2). More specifically, when no data pulse is present (FIG. 1, V=0 for t&lt;t.sub.1), the light source emits a DC light amplitude designated L.sub.o in FIG. 2. For a leading edge, upward transition of a data pulse (FIG. 1, from 0 to V.sub.o in t.sub.1 to t.sub.2), the light amplitude doubles (2L.sub.o) for a relatively short time .tau.&lt;T (FIG. 2) and then returns to the DC level L.sub.o. For a trailing edge, downward transition of the data pulse (FIG. 1, from V.sub.o to 0 in t.sub.3 to t.sub.4), the light amplitude decreases to zero (or near zero) for an equal time .tau. and then returns to the DC level L.sub.o.
At the receiver of the asynchronous system, the transitionally encoded light signal is converted to an equivalent bipolar electrical signal by a suitable photodetector. Thresholds are set up to detect the pulses of the bipolar electrical signal and logic circuits reconstruct the original binary electrical signal from the leading edges of the bipolar signal.
One prior art fiber-optic system of this type is described in U.S. Pat. No. 4,027,152 granted to W. W. Brown et al on May 31, 1977. The transmitter generates transitionally encoded light pulses and, in addition, a refresh light pulse of the same polarity as the preceding pulse whenever there has been no pulse for a predetermined amount of time. In the receiver shown in FIG. 4 of the patent, a peak detector 126 is used to provide automatic gain control (AGC) to the received signal which, in turn, maintains a constant signal amplitude at the input of a level shifting network (resistive ladder 110) and a comparator 114. In order to compensate for the level shifts due to the inherent offsets of the linear amplifiers, and to remove the DC component from the output of photodiode 100, a DC feedback network 134,135 forces the DC amplitudes of the differential outputs of the linear gain stage 104,108 to be equal. In order for the system to remain operative, the AGC amplifier 104 must remain active, which means that the peak detection circuit 126 must remain charged. This charging function is performed by the refresh pulses which do not alter the state of flip-flop 116 and, therefore, do not in principle interfere with the transmitted data pulses. In practice, however, we have found that the refresh pulses do interfere with the operation of the fiber-optic link. When a refresh pulse occurs at a data pulse transition, the transition time may be altered by as much as 15 nsec or more. This coincidence of refresh and data pulses results in a data dependent jitter and pulse width variation which is objectionable in some applications such as the proposed No. 4 ESS fiberoptic data link.